A prism sight or prismatic sight, sometimes also called prism scope or prismatic scope, is a type

telescopic sight A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a ''reticle'' – mounted in a focally appropriate po ...

which uses a

roof prism A roof prism, also called a Dachkanten prism or Dach prism (from German: ''Dachkante'', lit. "roof edge"), is a reflective prism containing a section where two faces meet at a 90° angle, resembling the roof of a building and thus the name. R ...

for its image-erecting system (instead of the series of

relay lens In optics, a relay lens is a lens or a group of lenses that receives the image from the objective lens and relays it to the eyepiece. Relay lenses are found in refracting telescopes, endoscopes, and periscopes to optically manipulate the light ...

es found in traditional telescopic sights). The use of prisms makes it possible to construct a shorter and lighter sight, or with an offset between the

eyepiece An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...

and

objective Objective may refer to: * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pronoun that is used as a grammatical object * Objective Productions, a Brit ...

axes Axes, plural of ''axe'' and of ''axis'', may refer to * ''Axes'' (album), a 2005 rock album by the British band Electrelane * a possibly still empty plot (graphics) See also *Axess (disambiguation) *Axxess (disambiguation) Axxess may refer to: ...

History

Prisms have been used in

periscopes A periscope is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position. In its simplest form, it consists of an outer case with ...

and

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

for a long time. Thomas H. Doughty of the United States Navy invented a prismatic periscope for use in the American Civil War (1861–1865), which was a further development of the mirror-based periscope invented by Marie-Davey in 1854. Binoculars with prisms appeared in the 1890s. During

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

, the

US Army The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, cla ...

chose to equip the

M1903 Springfield The M1903 Springfield, officially the United States Rifle, Caliber .30-06, Model 1903, is an American five-round magazine-fed, bolt-action service repeating rifle, used primarily during the first half of the 20th century. The M1903 was first u ...

repeating rifle A repeating rifle is a single- barreled rifle capable of repeated discharges between each ammunition reloads. This is typically achieved by having multiple cartridges stored in a magazine (within or attached to the gun) and then fed individually i ...

for the

sniper rifle A sniper rifle is a high-precision, long-range rifle. Requirements include accuracy, reliability, mobility, concealment and optics for anti-personnel, anti-materiel and surveillance uses of the military sniper. The modern sniper rifle is a por ...

role by mounting a prismatic sight developed by the

Warner & Swasey Company The Warner & Swasey Company was an American manufacturer of machine tools, instruments, and special machinery. It operated as an independent business firm, based in Cleveland, from its founding in 1880 until its acquisition in 1980. It was fo ...

. It was a short and compact sight, and the prisms allowed the objective to be angled to the side so that the 1903 rifle could be loaded with a clip from above. It was also still possible to use the iron sights with the scope mounted. The M1908 version of the scope had 6-times magnification, while the later M1913 version had slightly less magnification at 5.2×. The background for choosing a lower magnification was a larger

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Humans a ...

and improved light transmission. The sight was constructed of steel and brass, was painted black inside for optical performance, and had a relatively high mass of 1020 grams. The

eye relief {{Short description, Optical instrument The eye relief of an optical instrument (such as a telescope, a microscope, or binoculars) is the distance from the last surface of an eyepiece within which the user's eye can obtain the full viewing angle. ...

was only 38 mm, so the sight was equipped with a rubber

shield to prevent the scope from hitting the face during

recoil Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force requ ...

. The

reticle A reticle, or reticule also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the screen of an oscilloscope, ...

was a thin crosshair, as was common for this time period. These sights were also fitted to the

Hotchkiss M1909 Benét–Mercié machine gun The Hotchkiss M1909 machine gun was a light machine gun of the early 20th century, developed and built by Hotchkiss et Cie. It was also known as the Hotchkiss Mark I, Hotchkiss Portative and M1909 Benét–Mercié. Design It was based on a design ...

. Canadian Forces also ordered 500 samples of the 5.2× variant. Moisture on the inside of the lenses was one of several problems in the field, and the scope was known to loosen. In total, approximately 8,000 units of these prismatic sights were produced. The scope sight had the greatest magnification of the First World War, but in return only had a 4.5 degree field of view. A well-known later example is the fixed-magnification Trijicon ACOG which has been used in combat by the

US Marine Corps The United States Marine Corps (USMC), also referred to as the United States Marines, is the maritime land force service branch of the United States Armed Forces responsible for conducting expeditionary and amphibious operations through combi ...

and

USSOCOM The United States Special Operations Command (USSOCOM or SOCOM) is the unified combatant command charged with overseeing the various special operations component commands of the Army, Marine Corps, Navy, and Air Force of the United States Ar ...

. In the 2010s, several manufacturers including Trijicon, and Vortex began offering prism sights on the consumer market. On 2019 October 1, the Chinese

QBZ-191 The QBZ-191 Automatic Rifle () is a Chinese assault rifle chambered for the 5.8×42mm intermediate cartridge, designed and manufactured by Norinco as a new-generation service rifle for the People's Liberation Army (PLA) and People's Armed Polic ...

automatic rifle was formally revealed at the 70th Anniversary National Day military parade, and comes standard with a new 3× daylight prismatic sight called QMK152 and QMK-171A.

Construction

Most prism sights use roof prisms, by the same principle as compact handheld binoculars and

spotting scopes A spotting scope is a compact high-power telescope optimized for detailed observation of distant objects. They are used as portable optical enhancement devices for various outdoor activities such as birdwatching, skygazing and other naturalis ...

. The reticle is etched onto one of the prisms or lenses, making it easy to illuminate the reticle from the back of the sight, even when active illumination is turned off.

Features

A prism sight is a form of scope sight, and compared to a scope sight, much will therefore be similar.

Magnification

Prismatic scopes often have fixed magnification, such as 1× magnification (no magnification or "non-magnifying"), 2×, 3× or 4x, sometimes 5× or more. The low magnifications make them best suited for shooting at short or medium distances. There are also prism sights with adjustable magnification, such as the ELCAN Specter DR/TR series.

Reticle

Like scope sights, prism sights have an etched

which makes them suitable for shooters with

astigmatism Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at nig ...

, unlike

reflex sights In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

or holographic sights which may be near useless for these shooters depending on the severity of the astigmatism. Furthermore, the reticle is often illuminated, but is also visible without illumination (ulike reflex sights). Etched reticle also makes for the possibility to have more advanced reticles, and some prism sights are available with ballistic reticles which can be used for holdover for shooting at varying distances without turning the adjustment knobs.

Diopter

Prism sights usually have eyepieces with adjustable

diopters A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or curv ...

so that the image can be adjusted for

nearsightedness Near-sightedness, also known as myopia and short-sightedness, is an eye disease where light focuses in front of, instead of on, the retina. As a result, distant objects appear blurry while close objects appear normal. Other symptoms may include ...

farsightedness Far-sightedness, also known as long-sightedness, hypermetropia, or hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blurred effect is due to incoming light being focused behind, i ...

Parallax

Prism sights have a somewhat larger parallax error than red dot sights. Usually, they do not have an adjustable

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

like some scope sights.

Eye relief

Another disadvantage is eye distance.{{Cite web , title=Red Dots 101: Understanding the Types of Sight Systems , url=https://www.nrablog.com/articles/2017/2/red-dots-101-understanding-the-types-of-sight-systems/ , access-date=2022-08-08 , language=en Like riflescopes, the eye must be within a certain distance from the scope, unlike a

reflex sight A reflector sight or reflex sight is an optical sight that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view. These sigh ...

red dot sight A red dot sight is a common classification for a type of non- magnifying reflector (or reflex) sight for firearms, and other devices that require aiming, that gives the user a point of aim in the form of an illuminated red dot. A standard desig ...

") or

holographic sight A holographic weapon sight or holographic diffraction sight is a non- magnifying gunsight that allows the user to look through a glass optical window and see a holographic reticle image superimposed at a distance on the field of view. The hologram ...

References

Firearm sights